A sensor of an oxygen concentration cell type such as a zirconia sensor is generally known as a sensor for detecting incomplete combustion and is put into practical use in a combustor using gas as fuel. The zirconia sensor has certain characteristics wherein electromotive force suddenly changes at an air fuel ratio (hereinafter referred to as M value) M.ltoreq.1 as shown in FIG. 1 in a high temperature atmosphere, and the inner resistance reduces due to the temperature increase as shown in FIG. 2. Accordingly, it is used also as a sensor, with the use of the temperature characteristics, for detecting an accidental fire as well as the ignition of a fire. FIG. 3 shows one example of the detecting circuit. In the drawing, 1 is a zirconia sensor, which is made equivalent to the electromotive force e.sub.i and the inner resistance Ri. Resistor 2 is connected to a DC power supply 3 and in series with the zirconia sensor 1 and forms a voltage divider with the sensor device. At a and b are detection output terminals, respectively.
Before combustion, the inner resistance Ri becomes several hundred megohms, which is sufficiently larger than the series resistance 2, and the output electric potential becomes almost equal to the current power supply 3.
When the burner is operative, the inner resistance Ri lowers through the increase in temperature and becomes a value sufficiently smaller than the series resistance 2. The divided voltage potential through the series resistance 2 and the inner resistance Ri, therefore, drops as shown with a solid line A in FIG. 4 to an extremely small value. Accordingly, the detection of the drop in voltage can indicate that the burner has ignited. However, the conventional construction is uneconomical in power consumption as the voltage is normally applied directly to the sensor and the power supply circuit is required to impress the stabilized voltage to the sensor to positively detect the ignition because of a large dispersion of the inner resistance of the sensor so that a power-supply stabilizing circuit is required, thus resulting in complicated construction and higher cost.
As a method of solving such a problem, it is considered that voltage application to the sensor should stop and only the electromotive force e.sub.i of the sensor itself should be outputted. However, in this case, there is a problem wherein the output of the sensor will not suddenly change if the burner is ignited and the ignition detection becomes difficult to perform. Namely, although the sensor is heated with the combustion heat, when the burner is ignited, to start to generate the electromotive force of the oxygen concentration difference portion, the electromotive force change is slight, because the combustion is performed (the broken line A' range of FIG. 4) in the normal air fuel ratio zone (hereinafter referred to as a normal M value). Accordingly, there is no change in the sensor output, which makes it difficult to distinguish between the presence and the absence of the ignition.
A first object of the present invention is to ignite the burner, in a condition where the voltage application is not performed upon the sensor, in a range, i.e., a low air fuel ratio (hereinafter referred to as a low M value) smaller than 1 in the air fuel thereby to remove the above-described problems, that is, to ensure the ignition detection and to improve economy and cost.
Also, although carbon monoxide is produced in the case of the low M value ignition, a second object of the present invention is to reduce the production amount of the carbon monoxide as much as possible by the movement of the burner combustion to the normal M value simultaneously with the ignition detection.
Also, it is not desirable that much of the carbon monoxide which is harmful to the human body is contained in the combustion exhaust gas to be produced upon the ignition at the low M value. Accordingly, time required from the combustion start to the ignition detection preferably as short as possible. A third object of the present invention is to preheat the sensor to try to shorten the time required for the ignition detection.
In a type of voltage application to the conventional sensor, the temperature of the sensor falls when an accidental fire occurs for some reason, so that the inner resistance Ri becomes 100 megohms as before the combustion and the output potential abruptly rises as far as to an approximately equal value as that of the DC power supply 3 as shown with a solid line B of FIG. 4. Accordingly, the accidental fire in the burner can be detected through the detection of the voltage increase. However, although the changes in the electromotive force of the sensor itself are required to be used as an accidental signal in a type in which the voltage is not applied to the sensor, the electromotive force hardly changes as shown with a broken line B' of FIG. 4 when the burner is being burned in a normal M value condition of 1.0 or more in M value. Accordingly, it is difficult in the circuit design to detect it as an accidental fire through the electromotive-force difference even if the electromotive force of the sensor becomes zero due to the accidental fire of the burner, and error operations may often be caused even if the accidental fire is adapted to be detected.
Thus, a fourth object of the present invention is prevent voltage application to the sensor during the burner ignition. Rather, the voltage is applied to the sensor after the lapse of a given amount of time to enable the accidental fire to be positively detected.